Identified in 1985 as a novel oncogene, Dbl has become the prototype of an expanding family of guanine nucleotide exchange factors (GEFs) for the Rho family of small GTPases. GEFs activate small G-proteins by catalyzing the exchange of bound GDP for GTP. This important role in G-protein regulation, and the recognition that many GEFs cause cellular transformation, has generated great interest in these proteins. While much research has focused on the mechanisms by which Dbl family GEFs induce transformation through interactions with Rho family proteins, there is limited understanding of the mechanisms by which upstream stimuli transmit messages via these critical small G-proteins. Progress has been made in understanding the regulation of a few Rho GEFs, but little is known about the in vivo regulation of the dbl proto- oncogene (proto-dbl). Previous experiments revealed that truncation of the first 497 amino acids from the 115 kDa proto-Dbl resulted in a 66 kDa oncoprotein with 50-70 times its transforming activity. This finding suggests that the 497 amino acid N-terminal sequence plays an important role in the regulation of the guanine nucleotide exchange and transforming activities of proto-Dbl. The goal of this study is to elucidate the molecular mechanisms which regulate proto-Dbl. We will determine: (1) the specific region within the N-terminal domain which down- regulates transformation and its role in targeting the subcellular localization of the protein, (2) whether inhibition of GEF activity is through an intra- or intermolecular mechanism and identify molecules that interact with the N-terminal region of proto-Dbl, and (3) whether the mechanisms of N-terminal regulation of proto-Dbl are specific for the DH/PH domains of proto-Dbl by comparison with the GEF, Fgd1.